From BiblePortal Wikipedia

Charles Buck Theological Dictionary [1]

The aiming at our own interest only in every thing we do. It must be distinguished from that regard which we ought to pay to the preservation of our health, the cultivation of our minds, the lawful concerns of business, and the salvation of our souls. Self-seeking evidences itself by parsimoniousness, oppression, neglect, and contempt of others, rebellion, sedition, egotism, immoderate attempts to gain fame, power, pleasure, money, and frequently by gross acts of lying and injustice. Its evils are numerous. It is highly dishonourable and abasing; transforming a man into any thing or every thing for his own interest. It is sinful, and the source of innumerable sins; as perjury, hypocrisy, falsehood, idolatry, persecution, and murder itself. It is dangerous. It excites contempt, is the source of tyranny, discord, war, and makes a man a slave, and exposes him to the just indignation of God. The remedies to prevent or suppress this evil are these. Consider that it is absolutely prohibited.  Jeremiah 45:5 .  Luke 9:1-62 .  Hebrews 13:5 .  Colossians 3:5 . A mark of a wicked, degenerate mind; that the most awful curses are pronounced against it.  Isaiah 5:18 .  Habakkuk 3:9;  Habakkuk 3:12 .  Isaiah 15:1-2 .  Amos 6:1 .  Micah 2:1-2; that it is contrary to the example of all wise and good men: that the most awful examples of the punishment of this sin are recorded in Scripture; as Pharaoh, Achan, Haman, Gehazi, Absalom, Ananias and Sapphira, Judas, and many others.

Charles Spurgeon's Illustration Collection [2]

A certain king had a minstrel whom he commanded to play before him. It was a day of high feasting; the cups were flowing and many great guests were assembled. The minstrel laid his fingers among the strings of his harp, and woke them all to the sweetest melody, but the hymn was to the glory of himself. It was a celebration of the exploits of song which the bard had himself performed, and told how he had excelled high-born Hoel's harp, and emuted soft Llewellyn's lay. In high-sounding strains he sang himself and all his glories. When the feast was over, the harper said to the monarch, 'O king, give me thy guerdon; let the minstrel's mede be paid.' Then the monarch replied, 'Thou hast sung unto thyself, pay thyself; thine own praises were thy theme; be thyself the paymaster.' The harper cried, 'Did I not sing sweetly? O king, give me thy gold!' But the king answered, 'So much the worse for thy pride, that thou shouldst lavish such sweetness upon thyself. Get thee gone, thou shalt not serve in my train.' If a man should grow grey-headed in the performance of good works, yet when at the last it is known that he has done them all for himself, that he may be honoured thereby, his Lord will say, 'Thou hast done well enough in the eyes of man, but so much the worse, because thou didst it only to thyself, that thine own praises might be sung, and that thine own name might be extolled.'

Webster's Dictionary [3]

(1): ( n.) The act or habit of seeking one's own interest or happiness; selfishness.

(2): ( a.) Seeking one's own interest or happiness; selfish.