Acid grind is an inline skating term.Originally Posted by bmo
Definition for Acid grind CLICK HERE
Description of Acid grind CLICK HERE
Don't get your shorts (i.e., underwear) in a bunch (i.e., in knots/twisted all out of shape) means, don't get upset.
Note that, there are two possible origins: 1) placing one's hands in one's pockets so as to stabilize one's hands from reaching out at whatever is upsetting that person. b) muscles in the legs and buttocks may contract when a person is upset or nervous, causing one's 'shorts' to bunch up.