Lawyer has been used as a verb for a couple of centuries but the phrase lawyer up is of far more recent vintage (earliest cite 1995). Here’s the relevant parts of the entry in OED.
1. trans. To subject to a lawyer’s treatment or review.
1797 S. J. Pratt Family Secrets V. ii. 8 The Lord talked pretty much about swords and guns. Master Nicky Dabble‥seemed to think that lawyering him a little would be best.
2. intr. orig. U.S. Police slang. to lawyer up : to request a lawyer when being questioned by the police, often implying a probable lack of cooperation with the investigation; (also more generally) to hire a lawyer.
1995 N.Y. Times 23 Feb. c18/1 What really spooks the‥detectives on ‘N.Y.P.D. Blue’ is the prospect of a suspect ‘lawyering up’.