noun
secret account; alt account; anonymous account
Internet slang for a secondary social media account used anonymously or for private posting, separate from one's main account.
裏アカで本音を投稿している。
I post my true feelings on my secret account.
彼女は裏アカで趣味の写真をシェアしている。
She shares hobby photos on her alt account.
Compound of 裏 (うら, 'back, hidden') and アカ, a shortening of アカウント ('account'). The ateji 垢 is used for its reading あか.