フラグが立つ

Literally "a flag rises", or in more idiomatic English, "a flag has been set". See English flag (“computer science: true-or-false value, either recording the fact that a certain event has occurred or requesting that a certain optional action take place”).

• IPA^(key): [ɸɯ̟ᵝɾa̠ɡɯ̟ᵝ ɡa̠ ta̠t͡sɨᵝ]

フラグが立(た)つ • (furagu ga tatsu) godan (stem フラグが立(た)ち (furagu ga tachi), past フラグが立(た)った (furagu ga tatta))

1. (gaming) the prerequisite conditions for an event are fulfilled; something will happen
   • 2007, らき☆すた, episode 5:

     「夏(なつ)で祭(まつ)りで浴(ゆ)衣(かた)でしょ？これがギャルゲーだったら、フラグの一(ひと)つくらい立(た)つと思(おも)うんだけど」
     「フラグ？」
     「あとイベントシーンとかね。浴(ゆ)衣(かた)だし、野(や)外(がい)だし」
     「はいはい、そこまで」

     “Natsu de matsuri de yukata desho? Kore ga gyarugē dattara, furagu no hitotsu kurai tatsu to omou n da kedo”
     “Furagu?”
     “Ato ibento shīn toka ne. Yukata da shi, yagai da shi”
     “Hai hai, soko made”

     "It's a summer festival and we're wearing yukata, aren't we? If this was a galge, we'd have set some flag by now."
     "Flag?"
     "And gotten some event scene, too. I mean, we're literally outside in yukata."
     "Alright, that's enough."

   • 2007, “コンビニ”, cokesi (cokesi) (lyrics)‎:

     はにかんだ顔(かお)をして　こっちを見(み)ているよ！？
     これはフラグが立(た)ちそうな予(よ)感(かん)がしてきたよ

     Hanikanda kao o shite kotchi o mite iru yo!?
     Kore wa furagu ga tachisō na yokan ga shite kita yo

     Wait, she's looking my way with a bashful face—!
     It feels like something is going to happen (some flag is going to be set)

• ⇒ Chinese: 立flag

• Anime & Manga Stack Exchange - terminology - Why are romantic events sometimes called “flags”?