If you copy someone else’s copyright-protected teeshirt design - even if the design is a word design - you potentially can be sued for copyright infringement. If the words are trademark-protected, you potentially can be sued for trademark infringement.
But facts matter - and you have provided few of those.
Would you care to add some additional information? Thanks.
Definition of
plagiarism from Black’s Law Dictionary: The deliberate and knowing presentation of another person’s original ideas or creative expressions as one’s own.
Copyright infringement: An unauthorized use of a copyright-holder’s original and creative works that violate the exclusive rights held by the copyright holder. Exclusive rights include the right to reproduce (copy) the work, the right to create derivatives, the right to display the work, the right to perform the work, the right to authorize others to use the work.