taru-adjective, adverb taking the 'to' particle
Archaic reduplicated adjective/adverb describing something as thick or dense. Used in classical or literary contexts, often with the particle と.
In classical texts, it is sometimes used like 'a thickly overgrown forest'.
Reduplication of 厚 (あつ, 'thick') with the iteration mark 々, forming an archaic adjective/adverb.