Webster's Revised Unabridged Dictionary · 1913

Plead

Plead (plēd) , transitive verb

[Old English pleden, plaiden, Old French plaidier, French plaider, from Late Latin placitare, from placitum. See Plea.]

1.
To argue in support of a claim, or in defense against the claim of another; to urge reasons for or against a thing; to attempt to persuade one by argument or supplication; to speak by way of persuasion; as, to plead for the life of a criminal; to plead with a judge or with a father. [colloquial]
O that one might plead for a man with God, as a man pleadeth for his neighbor! — Job xvi. 21
2.
(Law) To present an answer, by allegation of fact, to the declaration of a plaintiff; to deny the plaintiff's declaration and demand, or to allege facts which show that he ought not to recover in the suit; in a less strict sense, to make an allegation of fact in a cause; to carry on the allegations of the respective parties in a cause; to carry on a suit or plea. — Blackstone
3.
To contend; to struggle. [Obsolete] — Chaucer

Plead , transitive verb

1.
To discuss, defend, and attempt to maintain by arguments or reasons presented to a tribunal or person having uthority to determine; to argue at the bar; as, to plead a cause before a court or jury.
Every man should plead his own matter. — Sir T. More

In this sense, argue is more generally used by lawyers.

2.
To allege or cite in a legal plea or defense, or for repelling a demand in law; to answer to an indictment; as, to plead usury; to plead statute of limitations; to plead not guilty. — Kent
3.
To allege or adduce in proof, support, or vendication; to offer in excuse; as, the law of nations may be pleaded in favor of the rights of ambassadors. — Spenser
I will neither plead my age nor sickness, in excuse of faults. — Dryden