A guess that is based on all the knowledge someone has, and is therefore as close to being correct as he or she can make it; the most likely deduction given the available information;.
Origin
Late 19th century.
Definition of best guess in US English:
best guess
noun
US
A guess that is based on all the knowledge someone has, and is therefore as close to being correct as he or she can make it; the most likely deduction given the available information;.