I learnt the Welsh expression Paid a gwgu! [paɪd a ˈgʊgɨ] from friends in Aberystwyth yesterday. It means ‘Don’t frown/glower/scowl!’. I like the sound of gwgu, which doesn’t seem like a frowny word to me – it’s more like a baby’s babbling. Related words include gwg (frown) and gwgus (frowning).
Words for frown in Irish, grainc and gruig, are possibly related to the Welsh word gwg. Other words frowny Irish words include púic and místá. The verbal expression is grain/gruig a chur ort féin (to put a frown on oneself), and an idiomatic way of saying, for example, ‘he frowned at me’ is bhí muc ar gach mala aige chugam (“he had a pig on each eyebrow to me”).