GIVES BIG F-U To Nick & Baby Mama

Iggy Azalea has a big 'ol message to Nick Young and the woman he knocked up -- EFF YOU BOTH!!

Iggy fired off the one finger salute -- which she clarified was directed at Nick and Keonna Green -- while at an airport in Australia on Wednesday ... hours after Keonna spilled her guts to US Weekly about her secret affair with the NBA star. 

Iggy also claims Keonna's a liar -- saying Keonna is simply not telling the truth about an alleged attempt to contact Iggy to clue her in on the adultery. 

Keonna -- who's 22 weeks preggo with Nick's child -- said she was hooking up with Nick since September ... 3 months after he proposed to Iggy. 

Yeah, we kinda get why Iggy's pissed ...