late 14c., compounen, "to put together, to mix, to combine; to join, couple together," from Old French compondre, componre "arrange, direct," and directly from Latin componere "to put together," from com "with, together" (see com-) + ponere "to place" (see position (n.)). The unetymological -d appeared 1500s in English by the same process that yielded expound, propound, etc. Intransitive sense is from 1727. Related: Compounded; compounding.