Originated July 21, 2018, when
BTS member V posted on WEIBO:
"紫爱你💜" which directly translates to
"I purple love you" or simply "I purple you" from Mandarin. The phrase orginally comes as "只愛你" which means "I only love you", but V replaced 只 (only) with the similarlly sounding 紫 (purple).
The wordplay is seen to be adamantly remarkable, considering even many chinese-speaking fans did not notice the pun at first, and V is a non-native speaker in English and Mandarin, both languages which of are necessary to make/understand this pun.
Because of this, the phrase "I Purple You" has now been popularized and is even used outside of BTS and their fans.
V further defines Purple to be the color of
trust and long-lasting
love, because Purple comes last in the rainbow, following all other colors.
In short, "I Purple You" is slang for "I
trust you", "
I love you" and "I
will stay by your side".