This phrase is a condensation of the colloquial expression:
“If only I could tap and sell that.” -or-
“You could tap and sell that.”
The expression is derived from the practice of tapping a keg of beer, or cask of wine; –whereby a spigot valve is inserted into the side of the keg, in order to tap the beer.
The expression figuratively means: “To access something of value”
In the last decade, the expression has been further condensed to simply:
“You can tap that” -or- “I’d tap that”
It is often used in conjunction with a sexual connotation.
The phrase "I'd tap that" is slang, used to express the sexappeal of another. Often this phrase is used sarcastically, when the referenced chick is beat (ugly).
"Damn, look at those tits!" "I'd tap that!"
"Holy shit that chick is beat!" "LOL I'd tap that."