This also is quote mining, along with mistranslation.
First of all 'ALMAH" should be translated as 'Young Woman'.
Second of all, if you read the passages in surround this line, you can see that isaiah identifies the young woman as 'the prophetess' (see Isaiah 8:3), and the 'kjv' has 'insures she conceives'.. which is sort of a english ephanism saying that he had intercourse with her.
Isaiah then goes on and says the before this child of an age that he can say 'NO' (usually a todder at the age of 2), the King of Assyria will get kicked out of Juddah, and won't be a bother to king ahaz anymore. The child is a timing device.
He confirms that, by saying in Isaiah 8:18 that he and his sons are the signs from God.