adverb
not so much; not so often; not particularly
Used with a negative sentence to mean 'not so much/often/long/many' or 'not particularly'. The exact meaning depends on context.
最近はそうそう映画を見に行かない。
I don't go to the movies so often these days.
この店はそうそう安くない。
This shop isn't particularly cheap.
interjection
oh, yes!; that's it!; I remember!
Exclamation used when suddenly recalling something or realizing something you had forgotten.
そうそう、明日は会議があったんだ。
Oh, yes! There's a meeting tomorrow.
interjection
Used to strongly agree with or affirm what someone else has said.
「この計画は難しいね。」「そうそう、本当に大変だ。」
"This plan is difficult, isn't it?" "That's right, it's really tough."
Reduplication of そう (so, like that). The adverb sense developed from the idea of 'in that way' combined with a negative, and the interjection senses from emphatic repetition.