What 𘰦 means
It represents the sound du, not a translated word. Specialists reconstruct this value. The script was made to write Khitan. Tradition dates its design to around 925 CE; one scholarly catalogue lists a tomb inscription from 1053 as the earliest known small-script monument.
Tradition links the design to the Uyghur writing system, though the scripts look little alike. The small script reused some signs from Khitan Large Script, the other Khitan system, as sound signs. These are links between writing systems, not proof this exact glyph came from another sign.
Fonts can draw the glyph differently: a linear Khitan font, version 16.000, uses a Chinese book-printing style called Song/Ming, not attested in surviving Khitan texts. Khitan writing can appear in vertical columns read right to left or horizontally. A box may mean your font lacks the glyph.
Questions people ask
1Why does 𘰦 show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
How to type 𘰦
𘰦\18C26Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.