na-adjective, noun
Describes a situation, plan, or condition that feels precarious, unsafe, or likely to go wrong. Often used with a nuance of unease or foreboding.
この計画はちょっと剣呑だと思う。
I think this plan is a bit risky.
夜の一人歩きは剣呑だから気をつけて。
Walking alone at night is dangerous, so be careful.
危ない is the most common word for 'dangerous' and is used in everyday speech. 剣呑 is less common and carries a slightly more literary or old-fashioned feel, often implying a sense of lurking danger or insecurity.
危険 is a straightforward noun/adjectival noun meaning 'danger' or 'peril'. 剣呑 is more subjective and emphasizes the feeling of risk or precariousness rather than objective danger.
The origin is uncertain. The kanji 剣 (sword) and 呑 (drink, swallow) are likely ateji used for their sounds rather than their meanings. The word may be related to the mimetic adverb けんのんと, suggesting a sense of precariousness.