The horizontal strokes are usually said to symbolise Heaven, Earth and Man, which are joined together by the vertical (omniscient) stroke.
However, this explanation is actually misleading, because originally the character depicted a large battle axe.
The meaning was 'big gun', or 'big shot', which came to mean powerful leader, and eventually king.
Mnemonic: KING WITH AX RULES HEAVEN, EARTH AND MAN
大様 (oosama) king
女王 (joou) queen
(Based upon Henshall, 1998)