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1.
Used to describe that a child is not yours. Thus, making you immune to child support.

Can be traced back to as early as greek mythology.
Zeus or Posiden had a baby with Hoecles who had a son named Illegitimacus

Since Hoecles cannot prove that Zeus or Posiden are the father, Illegitimacus is therefore Illegitimate, meaning none of them have to pay child support
by italian-props to wops- September 17, 2006
 
2.
A straight up whore fucking any dick in sight majorly without protection, leaving her with at least a dozen children each with no fucking clue who their daddy is.
1. Aint Shequiesha having yo baby? Hellz naw that bitch illigitimate.

2. Yo nig which bitch is yo illegitimate Lafonda or Shamonda?
by El Zacko September 05, 2006
 
3.
To get messed up beyond belief.
Drunk, high, etc.
Aye, lets go get illegitimate as hell this weekend mayne.
by Michael baganz April 30, 2007