Green’s Dictionary of Slang

get back! excl.

[the image of holding back people crowding to view something special]

(UK Und./US campus) an expression of admiration.

Show quotations
[UK]Leaves from Diary of Celebrated Burglar 56/1: As for jewelry — get back! — bought, borrowed or stolen, ’twas thar!
[US]Eble Campus Sl. Mar.
[US]Eble Sl. and Sociability 56: Many phrases formed around get are fixed expressions that show disbelief [...] or admiration (get back, get off, get up).