taru-adjective, adverb taking the 'to' particle
piercingly cold; bitterly cold
Literary, somewhat old-fashioned expression describing intensely cold weather or atmosphere. Often used with と (寒気凛々と) as a taru-adjective or adverb.
外は寒気凛々として、雪が降り始めた。
Outside it was piercingly cold, and snow began to fall.
寒気凛々たる冬の朝、湖面は凍りついていた。
On a bitterly cold winter morning, the lake surface was frozen over.
A four-character compound (yojijukugo) combining 寒気 (cold air/coldness) and 凛々 (severe, intense). The exact historical derivation is uncertain, but it follows a common pattern of intensifying a noun with a reduplicative adjective-like element.