Shortened form of "get the lead out of one's shoes" (or one's pants). Here, "lead" refers to the metal that is very heavy, thus the expression means roughly "get rid of whatever is slowing you down and hurry up".
Verb
gettheleadout (third-person singular simple presentgets the lead out, present participlegetting the lead out, simple pastgot the lead out, past participle(UK)got the lead outor(US)gotten the lead out)