A man that gives a woman needless amounts of affection or attention where the same is not reciprocated. This can be in the pursuit of pussy, female attention, or to satisfy his own dillusions that she actually cares about him.
Traditionally a "nice guy" would be viewed as a simp. However a simp extends beyond this to include men supporting or defending women they've never personally interacted with.
Despite already being friendzoned, Doug is a simp who continues to hang out with Stephanie because "she's so cool and he's happy just being friends with her".
During the twitch stream, a bunch of simps donated unreasonable amounts of money to the e-girl.
Tom wants to sleep with Kathy so bad, he was being a huge simp by backing up everything she said, regardless of whether or not he agreed with her.