Flowers given to a woman by her husband or boyfriend to make up for something that he is guilty of, but she doesn't know about. Usually, guilt flowers are given by the man without any apparant reason and as such are accompanied by a card that says something like "Just because". When actually, the pang of guilt after he cheated on his partner, or when he chose not to tell her about the visit to the strip club with the guys, or whatever sin he committed, overcame him and he felt he had to make it up to her, while not having to confess to anything.
Sam: I met this hot woman on the elevator at work this morning. Something came over me and the next thing I knew I'm asking her for her number. Jennifer will never know about it, but still, I feel bad I did it.
Jason: No worries, dude. Just pick up some guilt flowers on the way home tonight. It's like being forgiven without having to confess.
Sam: No way, won't she be suspicious?
Jason: Are you kidding? Women go crazy over flowers. By the time the flowers are dead and the spell wears off, it'll all be history. Trust me. Works every time.