What is "uath"

Uath, Old Irish Úath, hÚath , is the sixth letter of the Ogham alphabet, ᚆ, transcribed in manuscript tradition, but unattested in actual inscriptions. The kenning "a meet of hounds is huath" identifies the name as úath "horror, fear", although the Auraicept glosses "white-thorn":

comdal cuan huath (.i. sce L. om); no ar is uathmar hi ara deilghibh "a meet of hounds is huath (i.e. white-thorn); or because it is formidable (uathmar) for its thorns."

The original etymology of the name, and the letter's value, are, however, unclear. McManus (1986) suggested a value [y] (i.e. the semivowel). Peter Schrijver suggested that if úath "fear" is cognate with Latin pavere, a trace of PIE *p might have survived into Primitive Irish, but there is no independent evidence for this.