To think something is bigger, better, more valuable, or more likely than it actually is.
From the prefix 'over-' meaning 'too much' combined with 'estimate' from Latin 'aestimare' meaning 'to value or assess.' The compound verb emerged in English to describe excessive valuation.
Psychologists call the tendency to overestimate our own abilities the 'Dunning-Kruger effect,' and studies show that the less competent someone is at something, the more they tend to overestimate their skill in it.
Complete word intelligence in one call. Free tier — 50 lookups/day.