I have a great idea for a tee-shirt – can I register it as a trademark???
Short answer – probably not. The main issue with whether a slogan or word operates as trademark is how it is used. This is especially true of tee-shirts.
If the mark / slogan is just printed on a shirt – no protection – the Trademark Office deems that as being “merely ornamental” (there is an exception when the mark is also used for other products or business name). However if the mark / slogan is also used to identify the tee-shirt company, it may serve as a trademark and be registered (i.e. it is a “source identifier”) – the only caveat is that you would need to show it being used in that manner rather than just emblazoned on the front. Showing the mark this way can be done be showing the phrase on things like the name of an online store, packaging, an inside-tag, or a hang-tag.
So “Peanuts for Everyone” emblazoned on the front of a shirt – likely not a trademark. PEANUTS FOR EVERYONE brand tee shirts, with slogans like “Peanuts for Everyone” on the front – trademark.
With that said, if “Peanuts for Everyone” is included in a design on the shirt; that particular design may have copyright protection, but the phrase by itself is unlikely to retain the protection.