As for
"deuce"
(when both players have 40) that comes from the French "
à
deux" indicating that two points must be won consecutively
to win the game.
The word for a nil score
"love" originated and was popularised
by a very good early player from east London, who frequently took
his girlfriend to play tennis. His girlfriend was not a very good
player at all and rarely scored a point. Being the only player in
this pair who knew the way to score a game he would shout after
each point, 15 nil love, 30 nil love and so on.
Over time the nil was dropped and "
love" was adopted as
the term for a nil score. The other theory is that it came from
the French word
oeuf, which means egg. The oval shape of
the egg is symbolic of the numeral "0."