spend a penny

VERB
13 LETTERS · STARTS WITH S

spend a penny is a valid 13-letter word worth 19 points in Scrabble and 23 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
S1P3E1N1D2A1P3E1N1N1Y4
1 + 3 + 1 + 1 + 2 + 0 + 1 + 0 + 3 + 1 + 1 + 1 + 4 =19 PTS
DOUBLE WORD → 38TRIPLE WORD → 57
MEANING

What does spend a penny mean?