Forget, Forgetful - Vine's Expository Dictionary of New Testament Words
Forget, Forgetful
[ A-1,Verb,G2990, lanthano ]to escape notice," is translated "they (wilfully) forget" in 2 Peter 3:5, RV, lit., "this escapes them (i.e., their notice, wilfully on their part)," AV, "they willingly are ignorant of;" in 2 Peter 3:8, RV, "forget not," lit., "let not this one thing escape you" (your notice), AV, "be not ignorant of." See HIDE, IGNORANT, UNAWARES.
[ A-2,Verb,G1950, epilanthanomai ]
"to forget, or neglect" (epi, "upon," used intensively, and No. 1), is said
(a) negatively of God, indicating His remembrance of sparrows, Luke 12:6, and of the work and labor of love of His saints, Hebrews 6:10;
(b) of the disciples regarding taking bread, Matthew 16:5; Mark 8:14;
(c) of Paul regarding "the things which are behind," Philippians 3:13;
(d) of believers, as to showing love to strangers, Hebrews 13:2, RV, and as to doing good and communicating, Hebrews 13:16;
(e) of a person who, after looking at himself in a mirror, forgets what kind of person he is, James 1:24.
[ A-3,Verb,G1585, eklanthanomai ]
"to forget utterly" (ek, "out," intensive), is used in the Middle Voice in Hebrews 12:5, of "forgetting" an exhortation.
[ B-1,Noun,G3024, lethe ]
"forgetfulness" (from letho, "to forget," an old form of lanthano, See A, No. 1; cp. Eng. "lethal," "lethargy," and the mythical river "Lethe," which was supposed to cause forgetfulness of the past to those who drank of it), is used with lambano, "to take," in 2 Peter 1:9, "having forgotten," lit., "having taken forgetfulness" (cp. 2 Timothy 1:5, lit., "having taken reminder"), a periphrastic expression for a single verb.
[ B-2,Noun,G1953, epilesmone ]
"forgetfulness" (akin to A, No. 2), is used in James 1:25, "a forgetful hearer," RV, "a hearer that forgetteth," lit., "a hearer of forgetfulness," i.e., a hearer characterized by "forgetfulness."