This expression comes from Hebrew. It's a deformation of the Hebrew word: "LO", which means: NO. In this form it's used as a word of amazement in the meaning of: really ??? (or You don't say...). As long as the Laaaaaaa sounds - it's better.
A: Maccabi team lost in the basketball game.
B: La.
Or:
A: In the USA the currency is Dollar.
B: Laaaaa...
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