A phrase which is said when two people are thinking the same thing at the same time. It is generally only applicable if the thought is semi-random but can optionally also be used when the similar thought is obvious.
When used in online conversations, the inherent delay in the conversation can result in both people stating their thought at the same time but before receiving the other person's message. Both people may then say "Hmm La!", resulting in a double hmm la (or potentially an infinite cycle of hmm las)
Friend1: I feel like going home, this party is shite.
Friend2: Hmm La!