Altogether means completely, or counting everything. Not altogether true, ninety altogether, altogether different.
It is one word for these senses and two for another: all together means everybody in one place, and altogether means entirely — the pair is a standard trap. In the altogether is an old joke phrase meaning naked.
1adverbaltogetherCompletely, or counting everything.
“The meal cost thirty pounds altogether, including drinks.”
altogether is a degree.
Middle English altogeder is all plus together, written as one word from early on.
Middle English
altogeder
English, today
altogether
From Middle English altogeder, altogedere, equivalent to al- + together. Cognate with Scots awthegither (“altogether”), Middle High German alzegater (“altogether”), Dutch altegaar. Compare also Old English ealġeador, eallġeador (“altogether”), West Frisian allegearre (“altogether”). More at together. The noun sense nakedness was popularized in George du Maurier's 1894 novel Trilby.
Etymology from Wiktionary, CC BY-SA