There aren’t many English words containing the letter combination “uu”. In fact, only 3 of them are used in common speech:
continuum /kənˈtɪnjuəm/ (kən-tin-yoo-əm), rarely written also “continuüm”, is something without gaps or jumps. For example, languages often form a language continuum, which means that it is impossible to find sharp boundaries between different languages or dialects; they change continuously.
vacuum /ˈvækjuəm/ (vak-yoo-əm) is a space where no matter is present.
muumuu /ˈmuːmuː/ (moo-moo) is a loose dress of Hawaiian origin.
The other words are either obsolete or used in very specific areas, but nevertheless, they are considered part of the English language:
carduus /ˈkɑːdjuəs/ (kaa-dyoo-əs) UK, /ˈkɑːrdjuəs/ (kar-dyoo-əs) US is a genus of thisles (see an example on the right)
duumvir /djuˈʌmvə/ (dyoo-ʌm-və) UK, /duˈʌmvɚ/ (doo-ʌm-vr) US is one of two people jointly holding power. A duumvirate (pronounced as “duumvir” + “it”) is an alliance between two equally powerful leaders.
equus /ˈɛkwəs/ (ek-wəs) is a genus of animals that includes not only horses, but also donkeys, zebras, and other similar animals.
fatuus is used in the phrase “ignis fatuus” /ˈɪɡnɪsˈfætjuːəs/ (ig-nis fat-yoo-əs) borrowed from Latin, literally “foolish fire” (“ignis” = fire), which means “will-o’-the-wisp”, or generally a delusion, a false hope.
lituus /ˈlɪtjuːəs/ (lit-yoo-əs) was a Roman ritual wand, and the word is still used in mathematics to refer to a certain type of curve.
menstruum /ˈmɛnstruəm/ (men-stroo-əm), rarely written also “menstruüm”, is an archaic term for a solvent, i.e. a liquid that dissolves a solid.
mutuum, usually pronounced /ˈmjuːtʃuəm/ (myoo-tchoo-əm), is a legal term meaning “the loan for consumption”.
Nuuk /nuːk/ (nook) is the capital of Greenland.
obliquus /əbˈlaɪkwəs/ (əb-lie-kwəs) is a medical term for a muscle running obliquely.
residuum /rɪˈzɪdʒuəm/ (ri-zidzh-oo-əm) or /rɪˈzɪdjuəm/ (ri-zid-yoo-əm), rarely also “residuüm”, means “residue”.
triduum /ˈtrɪdʒuəm/ (tridzh-oo-əm) or /ˈtrɪdjuəm/ (trid-yoo-əm), rarely written also “triduüm”, refers to a 3-day period of prayer.
Weltanschauung (written with a capital W) /ˈvɛltɑnʃaʊʊŋ/ (velt-an-shaoo-ung) is a German word meaning “world view”. In English, it is used in cognitive philosophy in the meaning of a “wide world view”.