do·er \ˈdüə(r), ˈdu̇(ə)r, ˈdu̇ə\noun (-s) Etymology: Middle English, from don to do + -er — more at do 1.: one that does : one that performs or executes < a thinker or a doer — Sinclair Lewis > 2.: a domestic animal considered in terms of its capacity to respond to proper care and feeding — used with a qualifying adjective < those steers are very poor doers > < it doesn't pay to hold over any but your best doers > 3.Australia: eccentric