bartender (n.)

"tavern," 1590s, so called in reference to the bars of the barrier or counter over which drinks or food were served to customers (see bar (n.1)).

"person who tends another," late 15c., probably an agent noun formed from Middle English tenden "attend to" (see tend (v.2)); later extended to locomotive engineers (1825) and barmen (1883). The meaning "small boat used to attend larger ones" first recorded 1670s.