Quite simply, tentacles that originate from clothing. There are a number of permutations. They may appear to be normal clothes with the tentacles completely or mostly concealed, or the tentacles could be freely sprouting from the clothing. The the clothing itself may be made of tentacles. The tentacles may or may not be a part of/under control of the wearer; if so, the tags dark persona and corruption are often associated.
For tagging purposes, this tag encompass not just the "tentacle clothing" (触手服) pixiv tag, but also "tentacle suit" (触手スーツ) and "tentacle armor" (触手鎧).
