Michael Burnham and Sylvia Tilly wearing "DISCO" T-shirts
Mariner and Boimler wearing Chu Chu shirts
A t-shirt was a casual article of clothing, named for the T-shape of a torso with two sleeves. A t-shirt could have long or short sleeves, and could be worn either by itself or beneath or over another garment. T-shirts frequently had logos, slogans, or other symbols or words printed on them as decoration.
T-shirts were available for casual wear as part of the Starfleet uniform of the 2250s. Exercise clothes on the USS Discovery included T-shirts bearing the nickname "DISCO." (DIS: "Lethe", et al.)
In 2287, Captain James T. Kirk wore a long-sleeved t-shirt beneath a flannel shirt when camping in Yosemite National Park. When he was recalled to the USS Enterprise-A, he exchanged the flannel shirt for his uniform bomber jacket but kept the t-shirt, which bore the words "Go climb a rock." When the Chief of Starfleet Operations, Bob, saw Kirk's t-shirt, he remarked, "Well, we're dressing informally, aren't we?" (Star Trek V: The Final Frontier)
In 2380, Beckett Mariner and Brad Boimler wore Chu Chu dance t-shirts over their uniforms while attending a Zebulon Sisters concert on the USS Cerritos. (LD: "Terminal Provocations")
Captain Carol Freeman once wore a T-shirt with the word "RITOS" on it, similar to the "DISCO" shirts worn on the Discovery. (LD: "wej Duj")
When money and capitalism came to an end on Targalus IX, Beckett Mariner and Brad Boimler wore "No Cash Accepted" and "No Money, No Problems" T-shirts to the festivities. (LD: "Shades of Green")
While recovering from an induced coma, Doctor Hugh Culber told Paul Stamets he could not return to duty unless Stamets could spell "my partner brought me out of a coma and all I got was this lousy t-shirt." Stamets realized his mental capacity was still impaired, but was soon able to spell most of the phrase, missing only the hyphen in "t-shirt." (DIS: "Far From Home")