From BiblePortal Wikipedia

Fausset's Bible Dictionary [1]

So it ought to be read in  Matthew 12:24;  Mark 3:22;  Luke 11:15, etc. The Jews, in ridicule, changed Baalzebub, the Ekronite "god of flies", into Beelzebul, "god of dung" (which however is Zebel , as they changed Beth-El ("house of God") into Beth-Aven ("house of vanity"), when the golden calf was set up there. Zebul means "dwelling," lord of this lower world, "prince of the power of the air" ( Ephesians 2:2), and taking up his "dwelling" in human bodies ( Matthew 12:45).

Thus "master of the house" and "master of the dwelling" (Beelzebul) stand in happy contrast ( Matthew 10:25). As the Ekronite god was applied to by Ahaziah to east, out his disease, so the Jews taunted Jesus as using the same idol power to east out demons. Idols and demons, moreover, had a close connection ( 1 Corinthians 10:20-21). Beelzebul was thought to be the foul prince of both.

Bridgeway Bible Dictionary [2]

Jews of New Testament times used ‘Beelzebul’ as a name for Satan, the prince of demons ( Matthew 10:25;  Matthew 12:24-27). It was a variation of the name Baal-zebub, a Baal god whose home (according to an ancient Canaanite belief) was in the Philistine town of Ekron ( 2 Kings 1:2). The name meant ‘lord of flies’, probably because the local people believed this god gave the citizens of Ekron protection against disease-carrying flies that plagued the area.

By contrast other people interpreted the name in a bad sense – lord of flies, and therefore lord of filth. This was the meaning the Jews had in mind when they used the name as a title for Satan. Satan was Beelzebul, for he was lord of all things unclean, in particular unclean spirits, or demons ( Mark 3:21-22;  Luke 11:14-15). (Concerning the Jews’ accusation that Jesus cast out demons by Beelzebul, see Blasphemy .)

Smith's Bible Dictionary [3]

Be-el'zebul. (Lord Of The House). The title of a heathen deity, to whom the Jews ascribed, the sovereignty of the evil spirits; Satan, the prince of the devils.  Matthew 10:25;  Matthew 12:24;  Mark 3:22;  Luke 11:15 ff. The correct reading is without doubt, Beelzebul , and not Beelzebub .

Kitto's Popular Cyclopedia of Biblial Literature [4]

Beel´zebul, the name assigned ( Matthew 12:24) to the prince of the demons. There is no doubt that the reading Beelzebul is the one which has the support of almost every critical authority; and the Beelzebub of the Peshito (if indeed it “is not a corruption, as Michaelis thinks), and of the Vulgate, and of some modern versions, has probably been accommodated to the name of the Philistine god Baalzebub. Some of those who consider the latter to have been a reverential title for that god, believe that Beelzebul is a willful corruption of it, in order to make it contemptible. [BAAL].