The USMC has a long history, and therefore a lot of phrases, sayings and mottoes attached to it. The most common one seen in tattoos is semper fidelis (or semper fi for short), which means "ever faithful" in Latin.
"Leatherneck" and "jarhead" are terms applied to Marines because of the uniforms they have worn and the styles they have sported over the years. These words are good small tattoos for the arm, neck or head.
"Retreat, Hell! We Just Got Here!" is a great slogan showing the USMC's valor and pugnacity. It's attributed to a soldier fighting the Germans in World War I.
The unofficial mascot of the USMC is a bulldog, a dog known for toughness and coiled power. This relationship originated in World War II when English troops called the American Marines "devil dogs" and the name stuck. The US government put bulldogs on recruitment posters. Not surprisingly, a bulldog with a cigar in its mouth and a USMC helmet on its head works great as a tattoo. You could also make a devil dog by having an especially sinister looking bulldog breathing flames or smoke.
If you are currently in the USMC or are formerly a Marine, a tattoo signifying your rank is a great way to show your pride and dedication to the Marines. Of course, if you are still working your way up the ladder, it may be best to wait under you become a gunnery sergeant or lieutenant before getting a rank tattoo, as you will have to cover it up if you get a promotion.
Tattoos can permanently memorialize someone or something that's important to you. The ranks, names or dog-tags of fallen Marines you fought with or knew would be a great tribute to both the soldiers and their families. If you have a family military tradition, the names of close family members who have served can show both your heritage and pride.
Get a tattoo of Marines raising the flag on Iwo Jima. Any image that represents a war or battle in which Marines fought shows your historical appreciation and respect for the USMC.