Probably just the word MILF in general.  I got teased a lot for having a MILF for a mom when I was in school.  So anytime I see it I can't help but remember all the comments my classmates would make.

When I was a freshman in high school I started playing baseball.  There was some light hazing and teasing from the older boys, but it always felt light hearted. I love the sport and had a great time during my first few months. 

My Mom would pick me up from practice and while she is a beautiful blonde woman, she didn't draw too much attention from the other boys.  Nobody ever made any comments about her.  However, one day my mom decided to do something that would cause me weeks of torment.  She got a boob job over a holiday break.

The next time my Mom came to pick me up from practice she had enormous tits.  Every boy on the team immediately zeroed in on this change. The fact that my thin blonde mom had gone from normal sized breasts to huge pornstar tits gave the other boys a lot of material to bully me with.  For the most part it was just joking comments about her being a MILF.  I could brush those off, but there were two really rough days.

The two worst moments where when one white senior loudly talked about what he wanted to do with my mom.  He graphically described laying her down and titfucking her.  He ended his little performance by miming cumming into my Mom's mouth. 

The other time was in the locker room.  A black student pulled out his huge flaccid cock, waved it at me, and said to me, "Yo ask your Mom to pick me up too, I got her something right here."  Even soft, his cock was significantly bigger than mine is hard.  I remember all the other boys laughing.  It's unfortunately one of my most vivid memories ever lol. 

Nothing ever came of it obviously, and due to teenage boys' short attention spans the bullying stopped after a couple weeks, but it was a defining couple weeks.  While I hated it at the time, it really stuck with me.  That night I shamefully beat off imagining the black student fucking my Mom with his huge cock.  For months the fantasy of my Mom getting fucked by the other boys was all I masturbated to.   I don't know if it's better or worse that my Mom never knew this all happened.