結構
けっこう
adverb, na-adjective
quite, fairly; fine, satisfactory; no thank you
See also:
決行 (carry out, go ahead)
1.
quite, fairly, rather
Indicates a degree that is more than expected. Similar to 'pretty' or 'quite' in English.
2.
fine, good enough, satisfactory
Expresses that something is acceptable or sufficient as it is.
結構 has multiple meanings that depend heavily on context.
As an adverb (sense 1), it emphasizes that something is more than expected. Often carries a nuance of mild surprise.
As a na-adjective (sense 2), it means fine or satisfactory. Used in polite contexts to express approval or acceptance.
As a polite refusal (sense 3), もう結構です is a standard way to decline offers. Note that this usage can sound blunt if the tone is not carefully managed.
Common patterns
Usage: The refusal sense (もう結構です) can sound cold or dismissive in certain contexts. Softer alternatives include 大丈夫です or いいです.