Gaw, watergaw, weather gaw, weather gall
a partial rainbow, a temporary break in bad weather, a sign of bad weather to come. It is also used figuratively to denote a forlorn hope.
John Veitch in The History and Poetry of the Scottish Border (1878) is in no doubt that his watergaw is a partial rainbow: The weather gaw or broken bit of rainbow above the horizon and perhaps the most famous literary reference to a watergaw, which comes from Hugh MacDiarmid (1925), is consistent with this meaning: I saw yon antrin thing, A watergaw wi’ its chitterin’ licht Ayont the onding [I saw that rare thing, a rainbow with its shivering light beyond the downpour].
However, there are other quotations which seem to describe something rather different. This one is strangely reminiscent of a Close Encounter of the Third Kind: Four hours before the battle the same spectators observed (about two in the afternoon) in the sky three small globes of light, which they took for what we call (in the north) a weather gall (R. Forbes The Lyon in Mourning 1748).
A writer in the Scots Magazine (May 1823) uses it to describe a temporary respite in the weather: We’ve a bit blink the day, but you’ll find it naething but a weather-ga’, and it appears in Blackwood’s Magazine (1819) as a harbinger of heavy snow: He answered, that he had seen an ill-hued weather-gaw that morning, and was afraid it was going to be a drift. Another example of a weather-gaw as a bad sign comes from The Transactions of The Rymour Club (1924): Ower Gourlay’s Hole the weather-gaw Gleams to warn the boats awa.