- \ \  Bloat has a confused and uncertain history. It seems first to have appeared on the scene in the 13th century as an adjective, blout, meaning ‘soft, flabby’, a probable borrowing from Old Norse blautr ‘soft from being cooked with liquid’. This occurs only once, and does not resurface until the early 17th century, in Hamlet as it happens, as blowt: ‘Let the blowt king tempt you again to bed’. This appears to be the same word as turns up slightly later in the century as bloat, its meaning showing signs of changing from ‘flabby’ to ‘puffed up’. Then in the 1660s we encounter bloated ‘puffed up, swollen’, which paved the way for the verb bloat, first recorded in the 1670s.\ \ It is not clear whether bloater  comes from the same source. Its linguistic ancestor is the bloat herring , which may perhaps have been given its name on the grounds that herrings preserved by light smoking are plumper than those fully dried.
Word origins - 2ed. J. Ayto. 2005.