Abarim Publications' online Biblical Hebrew Dictionary
The forms צוץ (sus) and ציץ (sys) yield a small cluster of words that at first glance appear to not have much to do with each other. But if we assume that beneath all these words lies a fundamental meaning of being semi-mobile (like flowers fluttering to and for on their stems) or even detachable (like flowers picked), these words are obviously related:
צוץ I ציץ
The verb צוץ (sus I) or ציץ (sys) means to bloom or blossom (Psalm 103:15, Isaiah 27:6, Ezekiel 7:10) and is on rare occasions also used to mean to shine or gleam (of a crown: Psalm 132:18). Note that a similar duality exists in the verb קרן (qrn), meaning to radiate or have horns.
The sole derivation of this verb is the masculine noun ציץ (sis), meaning flower or blossom (Numbers 17:23, Isaiah 40:7, Job 14:2, 1 Kings 6:18) or "shining thing", which denoted an element of the high priest's outfit (Leviticus 8:9, Exodus 28:36). Of course, it's quite possible that these things weren't shiny things but rather things that dangled or stood on feeble stems and could move semi-freely.
The verb צוץ (sus II) occurs only once, and it's not clear what it means (or even whether it's truly a separate verb). It occurs in Song of Solomon 2:9, where the groom appears to be peering through the lattice. The word translated with lattice is also a mystery, and this whole statement is extrapolated from the previous. It could mean something wholly else; we're clueless.
The masculine noun ציץ (sis) is identical to the previous one meaning blossom or flower, but it's formally not clear if and how it relates to the above. It's used solely in Jeremiah 48:9, apparently with a meaning of wing(s), to be given to Moab, in order for it to take off. Perhaps this word suggests that Moab should be given "detachability," like a flower which remains for a while, even after it's been detached.
The feminine noun ציצת (sisit) occurs in two contexts, and denotes a small bundle of hair: either a tassel on a garment (Numbers 15:38-39), or a lock of hair on one's forehead (Ezekiel 8:3). Again, these items denote items with signature semi-freedom.