Definition - an estimate usually made without adequate information
Guesstimate came into use in the beginning of the 20th century. The word is, rather obviously, a blend of guess and estimate. Some people look askance at blends, a class of word that has given us oft-disparaged words such as bridezilla and frenemy. However, the category of blends is also responsible for words such as anonymuncule (“an insignificant anonymous writer”) and futilitarian (“one who engages in futile pursuits”), so they are deserving of some respect.
”All the same,” declares the Troy (N.Y.) Times, “the census estimate is undoubtedly more reliable than Chicago guesswork.” Out here it is considered merely a census guesstimate.
— Chicago Tribune, 15 Apr. 1904