Outer means further from the centre. The outer wall, outer space, outer London.
It is a comparative with no positive behind it — there is no out to compare against — and its opposite is inner. Outer space begins conventionally at a hundred kilometres up, a line drawn for convenience rather than by nature.
1adjectiveouterFurther from the centre.
“The outer walls of the castle are much thicker than the inner ones.”
outer is a position.
Old English utera is out plus the -er comparative ending. Utter is the same word.
Old English
ūtre
Middle English
outre
English, today
outer
Built from
From Middle English outre, outer, outter, uttre, from Old English ūtre, ūtera, ūterra (“outer”), equivalent to out + -er. Compare German äußere (“outer”), Danish ydre (“outer”), Swedish yttre (“outer”), Icelandic ytri (“outer”). utter.or a normal doublet?
Etymology from Wiktionary, CC BY-SA