From Longman Dictionary of Contemporary English(one) for luck(one) for luckspokenADD used when you take, add, or do something for no particular reason, or in order to say that you hope good things happen → luck
Examples from the Corpus
(one) for luck• Certainly there may be the odd chart or graph thrown in for luck but the basic requirement is for high quality text.• By now the only part of Mr. Rainsford's brewery left standing was the taproom kept for luck like an old horseshoe.• You get three kisses for your birthday, and one for luck.• I gave him one for luck on the back of his neck.• One for luck, and farewell.• Is he the kind of man to lend you the rings for luck for a short time?• Now he gave it one last violent shake for luck.• An extra teaspoonful for luck could well be disaster. 