A watergaw is a truncated rainbow, see Chas Penny’s photo above. These are not often seen in other countries but we see them due to Scotland’s combination of bright sunshine slanting at a low angle between heavy clouds and a splash of rain.
Written in Lowlands Scots (a.k.a. Lallans) Hugh MacDiarmid's poem is below but this link has Hugh’s explanation in English. https://www.youtube.com/watch?v=jGTlgox5URc ‘Sae noo ye ken, son’.
The Watergaw, by Hugh MacDiarmidAe weet forenicht i’ the yow-trummle I saw yon antrin thing,A watergaw wi’ its chitterin’ lichtAyont the on-ding;An’ I thocht o’ the last wild look ye gied Afore ye deed!--There was nae reek i’ the laverock’s hoose That nicht—an’ nane i’ mine;But I hae thocht o’ that foolish licht Ever sin’ syne;An’ I think that mebbe at last I kenWhat your look meant then.----------------------------------------------Photo by Chas Penny