- "Undoubtably" is considered to be nonstandard English by some authorities (for example, Garner's Modern American Usage (2009)), and the term is seldom found in modern literary writing. The Oxford English Dictionary provides no examples of usage after 1513 and characterizes "undoubtably" as "? Obs.," wondering whether the term is obsolete. Nevertheless, many examples of its usage can be found in 20th- and 21st-century popular English and in contemporary academic journals.
undoubtable + -ly