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")
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")