means To buy something — to acquire it by paying money; also, the firm grip or leverage that lets a thing hold or move.

from From Old French 'porchacier,' to chase after, pursue, or strive to obtain — built from 'por-' (forth, forward) and 'chacier' (to chase, the same root that gives us 'chase' itself). So to purchase was once to hunt a thing down. Only later did the chase shrink into the simple act of paying. The 'grip/leverage' sense — as in a foothold or a tool getting 'purchase' — comes from that same idea of laying firm hold on what you've pursued.