Richard Kanka -- whose daughter, Megan Kanka, was murdered in the '90s by a sex predator who, at the time, wasn't legally required to identify himself in his neighborhood, and who Megan's Law is inspired by -- is fuming over MTS casually dropping his daughter's name in "Hiss" ... the diss track that kicked off this new feud between her and NM.

He tells TMZ that Megan dragging his late daughter's name into the expletive-riddled song aimed at Nicki is as disrespectful as it gets, from his POV -- and he notes that MTS actually appearing to shed light on the law, which she does, doesn't change his sour feeling.

Kanka tells us that using his daughter's name in such a graphic/offensive manner was highly offensive to the pain his daughter and family felt ... and while he says he hasn't heard the track with his own two ears just yet, he has read the lyrics -- and it's still a no-go for him.

For the record, the lyrics in question are: "These hoes don't be mad at Megan [referring to herself], these hoes mad at Megan's Law" -- a reference Meg was using to take a shot at Nicki's husband, Kenneth Petty, who is a registered sex offender in New York.

A bit of a morbid history lesson ... the bill that turned into Megan's Law came into effect a month after the brutal murder and rape of Richard's 7-year-old daughter -- devastation Richard still lives with to this day ... so, understandably, it's a deeply sensitive subject.

He does admit Megan's music (rap) isn't his cup of tea ... so the only way he found out about it was when one of his kids told him. RK says he still chose not to listen though -- and instead read the "Hiss" lyrics to see what exactly was said about his daughter. Like we said, the guy still takes offense over it ... even though some see MTS's use of it positively.

There's also this ... Richard tells TMZ that the song left such a bad taste in his mouth that he's actually considering looking into his legal options to see if Meg's track went too far.

Unfortunately for Richard ... there's probably not much he can do on that front. Artists have liberties when it comes to wordplay/referencing big moments historically -- which is exactly what Megan did here ... so even though he's mad, a threat of legal action feels empty.

Anyway, this beef has only escalated since Meg threw the first shot -- Nicki's fired back with her own diss track, "Big Foot," and people have mixed reactions to it ... good and bad.