Uath

Uath, Old Irish Úath, hÚath (IPA: [wəθ]), is the sixth letter of the Ogham alphabet, ᚆ, transcribed ⟨ʜ⟩ 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.^[1] McManus (1986) suggested a value [y] (i.e. the semivowel).^[2] 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.^[3]

Bríatharogam

In the medieval kennings, called Bríatharogam or Word Ogham the verses associated with Úath are:

condál cúan - "assembly of packs of hounds" in the Word Ogham of Morann mic Moín

bánad gnúise - "blanching of faces" in the Word Ogham of Mac ind Óc

ansam aidche - "most difficult at night"" in the Word Ogham of Culainn.^[4]

References

1. ↑ McManus, Damian (1991). A Guide to Ogam. Maynooth Monographs. 4. Co. Kildare, Ireland: An Sagart. p. 37. ISBN 1-870684-75-3. ISSN 0790-8806. The letter name, therefore, presents considerable difficulties but one can be reasonably certain that the value h which the manuscript tradition accords it is no more than a cosmetic solution to the problem created by the loss of the original initial consonant. 
2. ↑ McManus, Damian (1991). A Guide to Ogam. Maynooth Monographs. 4. Co. Kildare, Ireland: An Sagart. p. 36. ISBN 1-870684-75-3. ISSN 0790-8806. In my 1986 paper I suggested an original value /y/, noting the parallel with the Ogam distinction between vocalic and consonantal u (symbols 18 and 3 respectively), but I also pointed to difficulties associated with this suggestions (see further §5.11). 
3. ↑ McManus, Damian (1991). A Guide to Ogam. Maynooth Monographs. 4. Co. Kildare, Ireland: An Sagart. p. 37. ISBN 1-870684-75-3. ISSN 0790-8806. The kennings identify hÚath with Old Irish úath 'fear, horror' and it has been suggested to me by Peter Schrijver of the University of Leiden that if the latter is cognate with Latin pavere 'to be terrified', some trace of Indo-European /p/ might have survived into Primitive Irish in pre-vocalic initial position. If so this might explain the appearance of hÚath in a consonantal series but the evidence of Continental Celtic, where /p/ in this position is completely lost, does not support the hypothesis. 
4. ↑ Auraicept na n-Éces Calder, George, Edinburgh, John Grant (1917), reprint Four Courts Press (1995), ISBN 1-85182-181-3