adverb
(not) particularly; (not) especially; (not) very; (not) much
Used with a negative predicate to mean 'not particularly', 'not very', or 'not much'. Common in formal or written contexts.
この問題はさして難しくない。
This problem is not particularly difficult.
彼の話はさして面白くなかった。
His story was not especially interesting.
それほど is similar in meaning but more conversational; さして is more literary.
Derived from the verb 然す (さす, 'to do so') in its continuative form. The exact historical development is uncertain, but it became an adverb used with negatives.