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.
I imagined the meeting content by guesstimating through the wall from the voices next door.
I tried to guess his salary by rough estimation, but I was wrong.
推量 is a general term for conjecture or guess; 壁越し推量 specifically implies guessing with very limited information, as if through a wall.
当てずっぽう is a random guess without basis; 壁越し推量 suggests some indirect clues, though still uncertain.
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.