noun, noun or participle which takes 'suru'
laughing to oneself; solitary laughter
Refers to the act of laughing alone, often involuntarily or when recalling something amusing. Can be used as a noun or with する.
彼は思い出し笑いで、ひとり笑いをしていた。
He was laughing to himself, remembering something funny.
電車の中で突然
I suddenly laughed to myself on the train and felt embarrassed.
noun
shunga; erotic pictures
Obsolete term for erotic art, especially shunga. The cross-reference to 春画 indicates this is a historical usage.
See also: 春画
江戸時代には「ひとり笑い」が春画を指す隠語として使われた。
In the Edo period, 'hitoriwarai' was used as a slang term for shunga.