Undue influence means some unfair and improper conduct, some coercion from outside, some overreaching, some form of cheating and generally, though not always, some personal advantage gained. The key element is that the influence was so great that the testator, donor or party to the contract had lost the ability to exercise his/her judgment and could not refuse to give in to the pressure.
An agreement procured by the undue influence of one party is voidable at the option of the other party. Undue influence includes any influence by which the exercise of free and deliberate judgment is excluded. Undue influence is presumed until the contrary is proved when the relation of the parties is such that one is entitled to the confidential advice of the other, as in the case of solicitor and client, of a trustee and his cestui que trust, and of a parent contracting with a child who has just come of age. In other cases of confidential relationship, the party seeking to avoid a contract must prove undue influence. ….. [Sutton & Shannon on Contracts, 6th Ed. p. 17B.]
Burden of proof lies in the first instance on the party who raises the plea of undue influence. If that party proves that the other party was not only in a position to dominate his will, but that the transaction entered into was also unconscionable, then the burden of proof that he did not use his dominant position to obtain an unfair advantage over the other is shifted on to him. ….. [Srimati Bidhumukhi Bassaya vs. Srimati Sarala Sundari Dassya & anr 6 DLR 97]
See, Section 16 of the Contract Act, 1872 (Act No. IX of 1872); Section 171C of the Penal Code, 1860; (XLV of 1860) the Representation of the People Order, 1972 (P.O. 155 of 1972) art. 77.