This expression was coined by members of Shipton's expedition up Everest in 1951 and refers to the Yeti, an ape-like creature said to inhabit the Himalayan region of Nepal and Tibet.
I always thought the idea of a Yeti was very attractive and could not understand why it should be termed Abominable.
Abominable means unequivocally detestable; atrocious: exceptionally bad or displeasing; worthy of, or causing, abhorrence, as a thing of evil omen; odious in the utmost degree; very hateful; detestable; loathsome; or execrable. In other words - not nice!
It seems I had misinterpreted the word abominable as used by Shipton and Co. - as I suspect do most people - since it has another meaning. In the context in which he named the snowman, abominable simply meant excessively large.