To the eyes, this word brings a repeated duality expressed in multiple ways: there are two each of two letters; it is bookended by two vertical lines coming from two different characters; the u is a rotation of the n before it; and the one letter not part of any other duality is made of two detached parts: i, with its stroke and its dot – a point and an extent.
In saying the word, you make four syllables; the middle two are attached to their preceders with a high front glide, while the first and last start with the lips – as a stop or as releasing from rounding. The vowels are low and forward first, but then in the back half of the word move to high back and then to a neutral step into a held “dark” /l/ with the tongue high at back. The word starts towards the front of the mouth but ends pulled back.
The central consonant is written with two letters but is at most a quick touch with an off-glide, and for many speakers in many instances is really just a nasalization of the glide it releases – the tongue may or may not fully touch. And yet it is heard as the same sound whether it is made with the tip of the tongue curling up and touching or with the blade of the tongue simply pressing up and forward while the tip stays down.
That curvy, contorting /bajænjuwəl/ pronunciation has come about because of how English vowels have shifted over the centuries and how we’ve come to pronounce Latin words generally. Were the word said the Latin way, it would be /biannual/ – just as it’s spelled, but to English ears more like “be on new wall.”
Except that the Latin word wouldn’t be biannual at all. This word may be made from Latin parts, but it wasn’t assembled from them until the later 1800s. The Latin word for a period of two years was (is) biennium, which lends to the word biennial, “every two years”. One might imagine that something that happens every half a year is semiennial (and every year and a half sesquiennial), but those words aren’t to be found.
The sense of this word is, as we all know too well, also dual. Its use to mean “every two years” dates from 1884 or earlier. Its used to mean “half-yearly” (every six months) dates from 1870 or earlier. Dictionaries list the two senses next to each other. So which is the correct meaning? It seems that there is no reliable verdict, and the court of common usage is divided. It’s just the same with other words, such as bimonthly and the perhaps even more bothersome biweekly – all these bi- usages appear to have come forth in the latter half of the 1800s. I recommend avoiding using these words, preferring clearer phrases such as every two years and every six months – unless you want to be ambiguous.