Phrases are typically eligible for trademark. If the phrase is being used within a design, the design is copyrighted, but the actual phrase is not protected under the copyright. A separate trademark registration would be needed for the phrase.
It's generally considered difficult to trademark a phrase for use on clothing. The phrase usually needs to be a brand name or be part of the branding or marketing for the clothing company. If you're just using it as part of a design, you can still use it, you just may not be able to protect it by preventing others from using it as well.
You can look up existing trademarks here: United States Patent and Trademark Office
. If it's trademarked, unauthorized usage would be infringement. If not, you may be ok. It's a good idea to do some Google searching as well, just to see if it's in use even though it may not registered. It's also a good idea to consult with an intellectual property attorney.