noun
guesstimating; rough estimation through a wall
Literally 'estimation through a wall'; used for making a rough guess based on limited or indirect information, often in a humorous or self-deprecating way.
隣の部屋の話し声から、壁越し推量で会議の内容を想像した。
I imagined the meeting content by guesstimating through the wall from the voices next door.
彼の給料を壁越し推量で当ててみたけど、外れた。
I tried to guess his salary by rough estimation, but I was wrong.
Compound of 壁越し (through a wall) and 推量 (conjecture). The exact origin is uncertain, but it likely arose as a humorous expression for making guesses based on faint or indirect evidence.