Alfred Adler, answered from the texts and cited to the page.
Overcompensation is what happens when the pressure from below — the feeling of inferiority, the dread of being found unmanly, weak, beneath — does not simply drive a person toward a reasonable sufficiency but pushes him past it, into excess, into the exaggerated and often self-defeating forms of strength, dominance, and superiority-seeking that betray the very wound they were meant to conceal.
The clearest picture of it is in the neurotic's masculine protest. When the fundamental feeling of inferiority demands stronger reassuring compensations — because of an expected defeat or the suspicion of one — the compensation ensues by a strengthening of the readiness for conflict, striving toward the masculine feeling of superiority, often by circuitous and more abstract ways, revealing simultaneous and often contradictory traits.1
You see the neurotic swinging between exorbitant arrogance and sudden modesty, between roughness and mildness, courage and cowardice — not because he is genuinely both, but because each swing serves the same single purpose: to secure himself against defeat, to enhance his own egoistic ideal or disparage that of others.2 The contradiction is only apparent; underneath it, the movement is perfectly consistent, always toward the same fictitious goal of masculine superiority.
What makes it overcompensation rather than healthy striving is the pitch of the goal and the rigidity of the means. The ordinary person who feels a weakness and works to correct it is compensating; the neurotic who cannot tolerate any reminder of weakness, who must be above in every encounter, who arranges sham combats to prove himself before venturing the real test — he is overcompensating, and the excess itself becomes the symptom.3
The "will to seem" grows more and more accentuated in case of failure, constantly strengthening the abstract main line of the old guiding fiction, rather than loosening it.4 And here is what makes the circuitous routes so instructive: overcompensation need not look like aggression. The masculine protest makes use of the feminine role in order to attain its purpose — passive traits, masochistic arrangements, even apparent submission — all pressed into the service of gaining power over others.5
The masochist who says, in effect, "it is you who must do what I would have you do" is overcompensating as surely as the bully; the goal of superiority is identical, only the road is reversed.6 So when you ask what overcompensation is, the short answer is this: it is compensation that has lost its proportion to the task, because the felt inferiority beneath it was never honestly faced — only fled from, by a more and more strenuous fiction of being above it.
when the fundamental feeling of inferiority demands stronger reassuring compensations— because of an expected defeat or the suspicion of one, this compensation ensues by a strengthening of the readiness for conflict which now strives toward the masculine feeling of superiority, often by circuitous and more abstract ways, revealing simultaneous and often contradictory traitsThe Neurotic Constitution
traits of exorbitant arrogance and modesty, roughness and mildness, courage and cowardice, lust for power and submissiveness, masculinity and femininity, all of which are used to gain security from defeat or in order to permit him by circuitous ways to enhance his own egoistic ideal or disparage that of othersThe Neurotic Constitution
sham combats play a great role; they are begun so that the neurotic may exercise himself, that he may gain experience from other or similar conditions, so that he may become more cautious, and in order to obtain proof from example that he dare not venture upon the main battleThe Neurotic Constitution
Without the help of the psychotherapeutist who in rare cases may prove to be a substitute for the fortunes of life, this 'will to seem' becomes more and more accentuated in case of failure, and constantly strengthens the abstract, main line of the old guiding fiction.The Neurotic Constitution
the masculine protest makes use of the feminine role in order to attain its purposeThe Neurotic Constitution
The masochistic attitude signifies: 'I am not governed by your power of attraction; it is you who must do what I would have you do.'Problems of Neurosis