When your partner is tenderlytonguingyour butthole (AKA you are receiving a rimjob) and you have the urge to pass gas, so you proceed to shart upon your partners nose and/or face, much to your twisted delight.
Alex: "So then what did Ashley do?"
Jimmie: "Well, she was blowing me, then the next thing I know, she's licking my butthole. I had to fart, so I countered her Rimjob with a Jimjob and spray-farted all over her face!"
Usually a verb, To Jimtom ones self is to fuck ones self over in the most serious possible way. Also a noun if someone is being a bit of a cunt
Max, "Did you hear about the guy that went shooting and blew his own balls off?"
Dean, "Yeah, he totally jimtommed himself."
Dean, "Max, I would just like to say...………. I have decided that I would like to make love to you."
Max, " Get to fuck and have a word with yourself, you Jimtom!!"