1: (hypothetical) used to express the desire to go back in time and change a single moment in your life, hoping that the new timeline will turn out better.

2: (altruistic) sacrificing yourself to satisfy someone else's needs whether the results benefit you or not. Used to describe literal sacrifices of life or happiness or simple compromises for someone you care for but don't agree with.

Comes from a scene in the fifth season finale (and sixth season premiere) of LOST wherein the character of Juliet--knowing that her death is imminent--quite literally taps the core of an atomic bomb, hoping to create a time paradox and better the lives of her friends. She does this even though not only will she die but also an alternate version of her would not ever meet her soulmate.

The term itself was coined by LOST creator and producer Damon Lindelof in the audio commentary track of the premiere episode on the first disc of the sixth season DVD set of LOST.
Damon: I really need to tap the bomb and not have married you.

Husband: I tapped the bomb for you and let you watch that cooking show when LOST was on.
Wife: Oh don't be such a martyr.

American Man: Uh...what are you doing to the core of that atomic bomb?
Japanese Man: I'm tappin' the bomb to try to go back in time and stop you assbutts from killing my family.
by Tav El December 22, 2010
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When a guy picks up a really hot slut and fucks her really hard.
"You should have seen me Tappin' The Gucci, she was froffin so hard, I was ballin out of control!"
by Mad Guru June 15, 2009
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An act performed by a male to gauge his partner’s responsive interest in having anal sex.

The male withdraws his penis during doggie style vaginal sex and begins to repeatedly tap the tip of his penis against his partner’s anus. If there is permission to enter, they will moan and grind hard on the tip assisting with rear entry. If anal is rejected, response is typically hearing “Oh hell no” or “Uh Uh” and tappin’ resumes on the vagina if you haven’t turned her off.
Finally got the booty last night with Rachel. Did a little tippintappin’ first to make sure.
by Eaton Holgoode January 29, 2018
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Whe you want to start off a new bottle of liquor right. Prior to opening every one who will be partaking in the 1st shot taps the "bottom" of of the bottle. Its almost like a luck thing or blessing the bottle.
The bartender opened a new bottle so everyone at the bar tappin' the bottle also took a shot.
by Momma luvs Cat December 25, 2016
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When you do something half way then throw it onto someone else
Hey man I'm tappin out, your it is in the sally
by Mr.BaddAss July 30, 2018
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Often used by law enforcement agents who do not trust there piece of shit handguns that often do not fire when needed. If the shit hits the fan and an agent does decide to tap and rack you will often see them smack the bottom of the firearm or the (magazine) and pull the slide on the top of the gun, then will begin to take aim at a punk, gangster, or even a zombie and proceed with feeding them lead.
Man did u see that cop? I'd say he had sleigh of hand on cause he was tappin and rackin like a mad man!
by Afghan Sam January 28, 2012
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