taru-adjective, adverb taking the 'to' particle
towering; soaring; lofty; majestic
Literary and archaic; used to describe mountains or other natural features that rise high and impressively. Often appears in classical Chinese-influenced prose or poetry. In modern Japanese, it is rarely used outside of set phrases or stylistic writing.
その山は巍々としてそびえ立っていた。
The mountain rose towering and majestic.
巍々たる峰々が連なる景色は壮観だ。
The view of the line of towering peaks is magnificent.
The exact derivation is uncertain; the kanji 巍 is used for its meaning 'high' or 'towering', and the reduplicated form 巍々 follows a classical Chinese pattern for intensifying adjectives. The reading ぎぎ is the on'yomi of the kanji.