mae gan foch bach glustiau mawr

• mae gan foch bach glustiau, mae clustiau mawr gan foch bach, mae clustiau gan foch bach

Literally "Little pigs have big ears".

mae gan foch bach glustiau mawr

1. (idiomatic) little pitchers have big ears (observation that small children often overhear more of what is said than adults realize or desire)

• R. J. Thomas, G. A. Bevan, P. J. Donovan, A. Hawke et al., editors (1950–present), “moch”, in Geiriadur Prifysgol Cymru Online (in Welsh), University of Wales Centre for Advanced Welsh & Celtic Studies