expression
to be honest; frankly; honestly
Fixed phrase used as a sentence adverb to preface a candid statement, often softening a potentially blunt or personal remark.
正直言って、その計画はあまり現実的ではないと思う。
To be honest, I don't think that plan is very realistic.
正直言って、彼の話は少し退屈だった。
Frankly, his story was a bit boring.
正直 is an adjective/noun meaning 'honest' or 'honesty'. 正直言って is a fixed adverbial phrase derived from it, used specifically to introduce a frank statement.
率直に言って is a similar phrase meaning 'frankly speaking', but 正直言って is more common in everyday conversation and carries a slightly softer, more personal tone.
A fixed expression combining 正直 (honest) and the te-form of 言う (to say), literally 'saying honestly'.