Mmm... i came across something intresting today.
Mr brown tries to explain the meaning of lah.
We use the "word", "lah" in our daily phrasings. But do we really know if it's "grammatically" right or wrong (in singlish terms)? yet it sounds right each time we use it.
why? why? why?

MR BROWN TRIES TO EXPLAIN THE MEANING OF LAH
The Meaning of Lah

OK, I cobbled this answer to your question from my own personal understanding and from linguistics studies conducted by people who study this sort of thing for a living, especially sociolinguist, Anthea Fraser Gupta, formerly of National University of Singapore, now at University of Leeds, UK.

So this answer is not entirely my work, but I copied and pasted parts which I thought best answered your question about "Lah", and tried to put it in layman's terms.

Your colleagues are wrong. "Lah" can be defined.

Part of the problem for native speakers of a language (if you consider Singlish a language dialect of a language), is that we are often not conscious of the language's rules and structure, even though we use the language correctly. The second problem arises when a term has multiple meanings, depending on context. So very often, a Singaporean can tell when "lah" is used wrongly, but cannot tell you exactly why.

Hence the difficulty for your colleagues to give you an adequate definition.


The Short Answer
The short answer is that "lah" is one of 11 particles used in Singapore English.

Hard to believe but, "lah" in Singapore Colloquial English actually has a linguistic definition and its use has an internal consistency and structure, just like any particle used in Standard English.

You cannot anyhow use lah, one. Or else Singaporeans will laugh at you.

"Lah" is what English professors call a Pragmatic Particle.

Definition of Pragmatic Particle:
pragmatic particle: A particle that may be used to express agreement or negation.


Definition of Particle:
A particle is a word that
- does not belong to one of the main classes of words
- is invariable in form, and
- typically has grammatical or pragmatic meaning.

Examples of particles in the English language are:
- to (in marking infinitives)
- up (in set up)
- not
- well
- oh


Definition of Pragmatics
Pragmatics is the study of the aspects of meaning and language use that are dependent on the speaker, the addressee, and other features of the context of utterance.


Singapore English Particles
Singapore English uses about 11 particles, mostly borrowed from Hokkien or Cantonese, to indicate attitude to what is being said. They work rather like "you know" and "you see". The three most common are ah (usually expects agreement), lah (strong assertion) and what (usually corrects something).

Here are some examples of particles used in Singapore English:

Lah:
"There's something here for everyone lah." ("There IS something here for everyone.")

"OK lah, bye bye." ("OK then, bye bye.")

"You see my husband's not at home lah. That's the problem, ah." ("You see my husband's not at home, you understand. That's the problem, you see.")

"Her price is too high for me lah" ("Her price is too high for me, I am afraid.")


Ah:
"Otherwise, how can be considered Singaporean ah?" ("Otherwise, how can this be considered Singaporean, don't you agree?")

"And then how many rooms ah?" (asking about a mansion/hotel… "So how many rooms does it have then?")


What:
"No parking lots here, what." ("Contrary to what you thought, you can see there are no parking lots here.")


More from Prof Gupta...

"Eleven pragmatic particles, loans from Southern varieties of Chinese, are used in Singapore Colloquial English. They express varying degrees of commitment to an utterance, and can be arranged on a single scale of assertiveness. They fall into three main groups: contradictory, assertive, and tentative.

"Singapore children acquire Singapore Colloquial English as a native language. These pragmatic particles are acquired early and without error.

"Each particle appears to have a wide range of multiple functions. These apparently disparate functions can be reconciled if the pragmatic particles are examined in terms of a system of marking degree of assertion, which results in different functions when the same particle is used in sentences of different types. No pragmatic particle in Singapore Colloquial English is associated with only one sentence type."

I hope my shameless paraphrasing and outright copy-and-paste of other people's studies plus my own little layman's thoughts, will help you understand the multi-faceted joy of using Singlish.