taru-adjective, adverb taking the 'to' particle
towering; lofty; high (as of a mountain)
Classical literary word describing something towering or lofty, especially mountains. Used with たる or と in the pattern 尭々たる/尭々と. Rare in modern Japanese outside of poetic or elevated prose.
The scenery of towering mountains stretching on was truly magnificent.
その塔は尭々とそびえ立ち、町を見下ろしていた。
The tower rose loftily, looking down over the town.
Alternate literary spelling without the repetition mark.
The exact derivation is uncertain; the kanji 尭 (high, lofty) is reduplicated to intensify the meaning. The reading ぎょう is the on'yomi of 尭.