The kanji 亜 resembles the character for 'two' (二) with an extra stroke extending downwards, visually suggesting 'secondary' or 'sub-' (as in 'second place' or 'below the main').
'A' — The reading 'あ' sounds like the first letter of 'ASIA', which this kanji often represents in compound words like 亜細亜.
Vocabulary
亜細亜アジア
Asia
亜熱帯あねったい
subtropical zone
東南アジアとうなんアジア
Southeast Asia
亜流ありゅう
derivative, imitation (often with a nuance of inferiority)