expression
I'm not so sure; I can't say for certain
Used when the speaker cannot assert something with confidence or finality. Often implies a degree of uncertainty or reservation.
彼が来るかどうかは言い切れない。
I can't say for sure whether he will come.
それが正しいとは言い切れない。
I'm not so sure that's correct.
More formal and direct, meaning 'cannot assert/declare'. 言い切れない is softer and more conversational.
Simply 'I don't know'. 言い切れない implies a judgment call where one hesitates to commit.
Negative potential form of 言い切る (to declare, to assert), literally 'cannot say decisively'.