The word "cwm" is Welsh (meaning vale, or glacial corrie). In Welsh, "w" is always a vowel, pronounced as "oo" or "u".

(In Welsh, "u" is usually pronounced as "i". "y" is pronounced either as an "i" or a "u".)

Where Welsh words have been borrowed by English, half the vowels have usually been discarded e.g. "glib", meaning slippery, insincere or superficial, derives from the Welsh "gwlyb" (f. "wleb"), meaning wet.