To work really hard on something.

Noun: a friend who is usually only called upon by a friend when a more primary/prominent friend is unavailable.

Taken from the term “second string” in an athletic competition situation. In football, if the star quarterback gets injured during a play, the second string is called off the bench to replace him/her. A “Second String Friend” is essentially benched until needed, if ever.

Synonyms: Plan B, Second Choice, Secondary Friend, Benched Until Needed

To bust ass is to fart. BUT if used with a possesive, it means to work hard.
Also often used with a possessive and immediately followed by an infinitive

when somebody lets out a disgustingly huge fart; this is usually announced.

To fall real hard on ones ass.

to rip an unprecdentedly large fart, one that sounds like a large plane has just flown overhead

also see busting ass

past tense: busted ass

A woman whose breast cleavage looks like someone's buttocks.