Noun
A board on which knives are cleaned or polished.
(historical, chiefly attributive) A way of arranging benches back to back, resembling a knifeboard; common on the open upper deck of early double-deck omnibuses and trams.
knifeboard seating
(historical, by extension) An early bus with narrow, restrictive seating; a knifeboard bus.
Source: en.wiktionary.org