One of the longest names in the Bible, Mahershalalhashbaz is actually a contraction of four Hebrew words: mahēr shalal ḥash baz (מַהֵר שָׁלָל חָשׁ בַּז), which means something like, "Quickly (to the) spoil; he hurries (to the) plunder."
The odd name, appearing in Isaiah 8:1 and 8:3, makes sense when you realize that it is another of Isaiah's prophecies involving the use of a child's name to warn people about potential threats. In the same way that Isaiah prophesied in 7:14–16 that a child would be born named "Immanuel" (meaning "God is with us"), and this was seen as a sign of Judah's deliverance from the threat of an alliance between Samaria (the capital of the northern kingdom of Israel) and Aram-Damascus, here, Isaiah has a son and is instructed by God to name him Mahershalalhashbaz, (meaning "Quickly (to the) spoil; it hurries (to the) plunder"), which Isaiah 8:4 says is a prophecy that, "the wealth of Damascus and the spoil of Samaria will be carried away by the king of Assyria" (Tiglath-Pileser III, r. 745–727 BCE). Essentially, just as the "Immanuel" prophecy set people's mind at ease about the threat to Judah from the north, so too was the Mahershalalhashbaz prophecy a prediction that Damascus and Samaria would be conquered by Assyria.
And in case you were wondering, the Assyrian King Tiglath-Pileser III conquered Damascus in 732 BCE, and his son, Shalmaneser V, died during the conquest of Samaria in 722 BCE, which was completed by Sargon II in 721 BCE.