noun, noun or participle which takes 'suru', intransitive verb
practicing by oneself; covert rehearsal
Slang term for practicing a skill or performance alone, often secretly or without telling others, to avoid embarrassment or to surprise them later.
発表会の前に、みんなに内緒でコソ練した。
Before the recital, I practiced secretly without telling anyone.
彼はいつも家でコソ練してからカラオケに行く。
He always practices at home in secret before going to karaoke.