shock of hair

Reader's Digest 6月号の記事 Ari & the Cello Chairからの抜粋です。
One day, while dashing between assignments, I mistakenly knocked on the wrong door in an office building. An elderly man with a shock of white hair opened the door, and there, behind him, was a serene tableau: a dark cello and a wooden chair with the design of a lyre on it.
"shock of white hair" ですが、何故こんな所に "shock" が出てくるのか不思議です。 "shock" を辞書で見るとありました、こんな説明が：
・Macmillan Dictionary: hair that is very thick: The man had a shock of white hair.
・Oxford English Dictionary: An unkempt or thick mass of hair: a slender man with an untamable shock of black hair
でも何故 "shock" にこんな意味がと思い語源辞典のOnline Etymology Dictionaryを見ると次の説明がありました。
"thick mass of hair," 1819, from earlier shock (adj.) "having thick hair" (1680s), and a noun sense of "lap dog having long, shaggy hair" (1630s), from shough (1590s), the name for this type of dog, which was said to have been brought originally from Iceland; the word is perhaps from the source of shock (n.2), or from an Old Norse variant of shag (n.). Shock-headed Peter was used in 19c. translations for German Struwwelpeter.
余りすっきりしませんが、"shaggy" + "thick" から "shock of hair" が生まれたと勝手に思うことにします。