Originally from Kuroko No Basuke; a person who blames themselves for everything, and constantly apologises for their existence.
Hannah: *picks up phone* Hello?
Peter: Hannah, I'm sorry I'm such a loser! Please can we get back together! I'm so sorry! Please forgive me I'm so sorry!
Hannah: I wouldn't have broken up with you if you weren't such an apologetic mushroom!
Peter: I'm sorry!
A person who is terrible at standing their ground when acting like a bitch. They lack the ability to own their attitude if it's too mean, unkind, or bitchy. It's regretful for them.
Refers to where someone noisily pretends to regret an action --- such as turning off a light, splashing water, etc. --- which caused someone else momentary shock and/or outrage, when in reality said prankster was completely intentional in performing said minor atrocity, and has gotten a huge giggle out of the hilarious-to-him distressed reaction of his "victim"!
Dude #1: flips the ceiling-light off in an otherwise-darkened garage where he is fully aware that his buddy is working
Dude #2: indignantly: Yo!!
Dude #1 in a classic "apologetic guffaw" tone "Huh-hoh --- saww-rreee" gives a big grin as he flips the switch back on again.