A sarashi is a long, winding strip of cloth, usually thick cotton or bandages, wrapped tightly around the midriff up to the chest. Historically, samurai wore them under their kimonos, to resist injury.
In anime series, a delinquent character, particularly in the classic older series, will be seen wearing one under their coat or unbuttoned shirt. The wrapping was a visible cue as to their general level of bad-assery. A tomboyish girl may wear one to bind and/or flatten her breasts, especially if she is trying to pass as male.
Alternatively, especially with kendo girls and other fighters, the wrapping may be a modified bra, wrapping only the chest area in a very nontraditional way while leaving the midriff bare, usually in a similar fashion to a tubetop (ie. post #4076845, post #3616420, post #2644312).
If the bandages do not cover the chest adequately, use the budget sarashi tag.
If the bandages are unwrapped, use undone sarashi.