Hamish

The origin of this form may be the genitive rather than the vocative. It was/is conventional in Gaelic speaking areas to identify people by referring to a parent, much as in English one might say "Jim's Colm". In Gaelic this would be "Colm Sheumuis", which would transliterated be "Colm Hamish". It's informal, a little different from a formal "Mac" surname. Subsequently, it may be that this form was intentionally used as a first name, in the same way that Scots people traditionally use surnames as given names. Alternatively, the informal patronymic may have been used as if it were a middle name, and subsequently used as a given first name by descendants who had not learned Gaelic. See also Mhairi.

It is absolutely correct to say that Hamish is from Sheumuis which is the vocative case of Seumus except for the vocative particle "a" has been omitted. [noted -ed]

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When talking to someone called Seumus one says "a Sheumuis!" which is pronounced /a haymush/. [a masculine Indo-European o-stem changes its "broad" final consonant to a "slender" final consonant] which in the case of s is like an English sh. But as sh is pronounced like an English h, another system has to be used. That system is based on the rule that a consonant in close contact with a front vowel [e or i] is pronounced slender. So s near a e I or I = sh sound. That is why Seán is pronounced Shawn.