/tuː/
[adverb] Used to mean 'also' or 'as well', or to show that something is more than necessary or wanted, such as 'too hot' or 'too many people'.
'Too' and 'to' were the same word for centuries — English speakers only slowly pulled them apart.
The word 'too' begins its life as a stressed form of the Old English preposition 'to', which carried the same basic meaning it does today — direction, purpose, and addition. In Old English, 'to' was doing enormous grammatical work, pointing toward places, goals, and recipients. But language has a way of needing emphasis, and when speakers wanted to stress the idea of 'in addition' or 'to a greater degree', they leaned harder on that little word. Over time, that stress became a signal of a different meaning entirely.
Through the Middle English period, roughly the 12th to 15th centuries, the spelling and pronunciation of the stressed form began to drift away from the unstressed preposition. Writers would sometimes spell it 'to', sometimes 'too', and sometimes 'too', all meaning the same thing depending on context. The doubling of the 'o' was a common medieval spelling trick to show that a vowel was long, so 'too' simply meant 'this vowel stretches out'. It was not a different word so much as a louder version of the same one.
The sense of 'also' or 'in addition' was the first distinct meaning to settle on the longer form. A speaker saying 'I want some too' was using the old idea of 'joining toward' something — moving oneself, figuratively, into the same group as others. This meaning is very old and appears clearly in texts from the 13th and 14th centuries, where 'too' begins to show up in writing as a recognisably separate word from the directional 'to'.
The sense of 'excessively' — as in 'too cold' or 'too fast' — developed from the same root idea of going further than something, pushing past a limit. If 'to' pointed you toward a destination, then 'too' eventually came to mean you had gone past it. This usage was well established by the 15th century and became the dominant meaning in everyday speech, giving the word a slightly negative flavour: 'too much' is almost never a good thing.
What makes 'too' quietly remarkable is how it shares its ancient DNA with so many other English words. The Old English 'to' is related to Old High German 'zu' and Gothic 'du', all descending from the Proto-Germanic root meaning 'toward'. Through Old English 'to' came not only 'too' and 'to', but also the number 'two' shares the same Proto-Indo-European neighbourhood — the root *dwo, meaning a pair, which fed into Old English 'twa' → Middle English 'two'. Even the prefix 'to-' in words like 'together' (Old English 'togædere') carries the same ancient pointing finger. 'Too' is, in a sense, a tiny word that has been doing important work in English for over a thousand years, growing heavier with meaning while barely changing its sound.