Or Garfield's response to a reprimand (Garfield is off panel for the whole strip):
Jon: Garfield! get your paw out of the cookie jar!
[next panel: no dialogue, Jon looks shocked]
Jon: Who taught you that gesture?!
Garfield: You, in the car, when you drive.
The Babes and Bullets special had Garfield fantasizing about being Film NoirPrivate Detective Sam Spayed. Most of it is spent flirting with very human women, and the ending involves a moment heavily implying he... drinks milk with one of them. Actual milk is present, of course, but Garfield's reaction to the invitation is pure Parental Bonus at work.
"Wanna pet my parrot?" (said to a woman). Even Jim Davis pointed out in a compilation how risqué that one is.
And then there was a strip in which Garfield got a lot of amusement out of scratching Odie's rear, because that made Odie become so excited that he jumped up and down. Garfield just kept scratching until poor Odie was in a frenzy. A few readers have noted that the gag looks suspiciously like a metaphor for something else....
When Jon first took him to see Liz, Garfield said that his uncle was an SRS patient.
I assumed he was referring to castration, but that's another plausible explanation.
Jon insults his brother perfectly when he sees Doc Boy is naked. Not shown, of course, but it still counts.
Before Liz and Jon started dating, Jon once asked her what she would suggest for an animal that's madly in love. She quipped, "I usually prescribe neutering." Even worse, in the 20th anniversary book, the strip is followed by a picture of Garfield eagerly holding up a pair of scissors.
That time Jon put his pants on when they came straight out of the dryer, and found out the hard way that certain metal parts might still be hot and thus very uncomfortable for a man to wear.
Look at the middle panel.◊ Garfield is choking a (rubber) chicken. Noisily. Even if you haven't heard of that euphemism, the way he's doing it is plenty suggestive. And Jon says he's going to "wring [his] rubber chicken's neck" in the next panel.
In In this strip Garfield says "I wonder if anyone will miss him" pointing to killing Nermal. Well he wouldn't kill Nermal. But it will be imaginable. You know decapitation.
Garfield's nod to an old-fashioned insult, being a roundabout way of calling someone a bastard.
Some of the strips collected in "Garfield from the Trash Bin" weren't finished or released due to pushing the boundaries of good taste. Most notably, one involves a man with a squirrel living in his pants who "appears to be storing..."
Another beach one, with Jon wearing a flotation device with a very Freudian placement of the head....which the woman he asked to "go boogieboarding" leaves in a much lower, more flaccid state when she turns him down....