Green’s Dictionary of Slang

(US) to take one’s first steps towards an objective.

a.2003

Ainzfern ‘A Strong Hold’ on 🌐 The two men were on the Commander’s sofa, ‘first basing it’ as Tom had laughingly referred to it. [...] Chakotay was grazing his way across Tom’s smooth, slender neck, running his hands over Tom’s chest, his fingers stopping every now and then to flick open another button on his shirt.