... but why lasta? and da tar tar?

One guess. A mispronounced "steak OF THE tar-tar". "OF THE" being used to announce the adjectival nature of tar-tar, which only becomes necessary as one learns *english one word ata time.

The "a" is pronounced like "uh" or "um", which may be a teacher passing their own 'pausing utterance' on to the attentive student. Just another bad guess.

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Dragging this back to shagging, by the ears...

What do you call 500 Indians with no nipples?

The Indian nippless 500