aa comes from Hawaiian ʻaʻā ('rough lava, to burn, blaze'). Adopted as a geological term for blocky, jagged basaltic lava, in contrast to the smooth pahoehoe. The path is chiefly one of transmission: an older form is taken into English, with the word's shape and meaning carried through the language names and forms preserved in the history. In modern use, the noun sense is "basaltic lava with a rough, jagged, clinkery surface", which keeps the etymology connected to the word's present dictionary value rather than treating it as a merely historical label. The wider word family includes related English forms such as pahoehoe, showing how the same base continues to support neighboring English vocabulary. The principal historical forms in the chain include Hawaiian ʻaʻā, which anchor the explanation in named linguistic material.