本気
ほんき · honki · JLPT N1 · noun
earnestness
本 (root, origin) + 気 (spirit, mind): the real spirit of a thing, so 'honki' means being genuinely serious, not joking.
ほんき is 'true' (本) plus 'spirit/energy' (気). Something done with real spirit is done in earnest, which is why 本気 means serious, truthful, or sacred.
本 — tree 木 + mark at base
本 is a pictograph of a tree (木) with a stroke at its base, marking the roots. From this 'root' meaning it extended to 'origin,' 'source,' and 'foundation.' In modern usage it also serves as a measure word for bound objects like books.