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Wiktionary
jump on

vb. 1 To board a vehicle. 2 (context idiomatic English) To attack verbally, to criticise excessively. 3 (context idiomatic English) To take immediate action regarding; to avail oneself of without delay.

WordNet
jump on

v. get on the back of; "mount a horse" [syn: hop on, mount, mount up, get on, climb on, bestride] [ant: hop out]