faint smile; half smile
A subtle, restrained smile that is not a full laugh; often used when someone is amused but trying not to show it, or when the situation is awkward.
See also: 薄笑い (うすわらい)
He got through the situation with a faint smile.
Her half-smile expression hid her true feelings.
Compound of 半 (half) and 笑い (laugh, smile). The exact historical derivation is uncertain, but the word transparently means 'half-smile'.