Kimashita wa

The phrase "kimashita wa" (キマシタワー) is often said when two females interact in a "more than friendly" way.

Originally, it came from the following text art related to Strawberry Panic:

　 ┏┓　 ┏━━┓┏━┓┏┓┏━━┓┏━━┓ 　 　 //.i:.:.:i:.:.:.:.:i,|:.i／iﾊ:.:./　ﾘ＼|､;ｲ:.:.:.:.i､:.:i….　　　 　 ┏┓┏┓┏┓
┏┛┗┓┗━┓┃┗━┛┃┃┃┏┓┃┃┏┓┃　 　 .| i.|:.:.;ｲ:.:.i:.:| |i　 ィ ∨　ゝ　ﾘ|/`ｲ:.:.:ﾄ:.Y､.　　　　　 ┃┃┃┃┃┃
┗┓┏┛　 　 ┃┃┏━┓┃┃┃┗┛┃┗┛┃┃┏━| i,|:.:.| i:.:.|i:.:|ｨｪｪク　　　 ｲﾐｪｭｬ.ﾚ:.:.i;,:.:） ヽ..━━┓┃┃┃┃┃┃
┏┛┗┓┏┓┃┃┗━┛┃┃┃┏┓┃　 　 ┃┃┃　 .i|.´i:.|　i:.:.iヽi /// ヽ　　　/// /:.:./､）　　　＼ 　┃┃┃┃┃┃┃
┗┓┏┛┃┗┛┃　 　 ┏┛┃┗┛┃┃　 　 ┃┃┗ ／ﾘ　i:.i .ﾄ:.:i:.ゝ　　ﾏ⌒ ､　　　/ｲ:.:.ifj´＼ i.　　ﾍ….┛┗┛┗┛┗┛
　 ┃┃ 　┗┓┏┛┏━┛┏┛　 　 ┃┃　 ┏┛┃　　`ｰ ＜i＼|＼ヽ 　　 ヽ_ノ　　　 /:.:.:/Y ヽ. ＼ ,／..　　┏┓┏┓┏┓
　 ┗┛ 　　 ┗┛ 　┗━━┛　 　 　 ┗┛　 ┗━┛　　　　　 ∨.|:.:.:`|`ゝ､　　　　 ,.／i:.:.:/ .i| ＼　 /….　　　 ┗┛┗┛

Of the character Tamao Suzumi. "Kimashita" is a past tense polite form of the verb "kuru," which means "to arrive" or "to come." "Wa" is an sentence ending indicating strong emotion, typically used by women but also sometimes men. In any case, the usage of "wa" at the end of sentence is indicative of a feminine tone.

This text art was afterwards frequently used on Japanese message boards like 2ch whenever a scene that feels yuri-like. This phrase has also been encountered in half-width katakana ｷﾏｼﾀﾜｰ and in an abbreviated form, simply "kimashi" (ｷﾏｼ).

Tamao Suzumi has actually never said this word in the show Strawberry Panic, so this was entirely a creation from the text art. The Tamao face that is depicted in this text art is actually what she looks like when she says "tamarimasen wa" (ﾀﾏﾘﾏｾﾝﾜｰ, "I can't stand it").

Usually when people use this meme, it's in subtle low-excitement situations. For example, you do often see phrase commented about scenes when two females kiss, but you see people use this phrase much more often in relation to scenes when one female compliments about another's appearance in a more-than-friendly way. In other words, people use this phrase when they perceive that yuri is about to begin, not when yuri is already happening.

This meme has also given birth to another related meme, "Kima City Tower," or "Kima-shi Tawaa" (キマシ塔). This of course is a pun.

(This information was taken from http://dic.nicovideo.jp/a/キマシタワー as well as various observations of comments posted on videos at Nicovideo).