noun
right beside; directly to the side
Emphasizes that something is immediately adjacent, not just nearby. Often used with に or の.
彼は私の真横に座った。
He sat right beside me.
真横の席が空いていた。
The seat directly next to me was empty.
横 is a general word for 'side' or 'beside'. 真横 adds emphasis, meaning 'directly to the side' or 'right beside', without any distance.
Compound of 真 (ま, 'true' or 'direct') and 横 (よこ, 'side'). The prefix 真 intensifies the meaning, giving 'directly to the side'.