This is dbu med aka headless aka cursive Tibetan script. There is not much information about this script online. Inspection of these charts and this page suggests to me that the word on this shirt is དབུགམ་ dbugs "breath", which is pronounced something like /uk/.
There is a flourish at the beginning and end; removing those gives you just the 4 characters and 1 vowel diacritic. I am not entirely certain what is up with the second vertical stroke.
The last vertical stroke is tseg (་) the punctuation used to separate syllables. Tibetan marks the ends of syllables, but does not mark the ends of words. This creates an interesting problem, as the Tibetan and Himalayan Library notes:
Another problem is that since punctuation only marks the boundaries of syllables, and not words, it is not entirely clear how to best combine syllables into separate words when transcribing Tibetan phonetically into Roman script.