Sill
A surname most likely from a nickname or pet form of Cecil, though it may also reflect older forms related to Silvester or Silvanus.
The #9,568 most common surname in the United States (2010 Census) — about 1 in every 90,888 Americans shares it.
Etymology
The surname Sill is best explained in the older surname books as a short form or pet name connected with Cecil, especially in medieval English usage. Harrison also suggests it may sometimes have been taken as a shortened form of Silvester or Silvanus, both ultimately from Latin names linked with woods or forested places. Because the sources do not agree completely, Sill should be treated as a surname with more than one possible historical source, though the Cecil connection is the strongest in the excerpts provided.
History
Bardsley treats Sill as a baptismal surname, meaning "the son of Cecil," and says the form was used to distinguish the masculine Cecil from the feminine Cecilia. The same tradition appears in related surnames such as Silcock, Sills, and Silkin. In modern use, Sill is comparatively uncommon in the United States, but it has persisted as a small English-language surname.
Origin
Regions
Variant spellings
Fun fact
Old surname writers thought Sill could be a nickname used to keep Cecil separate from Cecilia in everyday speech.
From the historical record
“O.E. wyt^, weor^, farm, etc.] SILL, a contr. of i Silvester, q.v. 2 Silvanus, q.v. 3 Cecil, q.v. IIller) = ®®"*''' ^^"^''' "i"- SILLIFANT, a West. Eng. form of Sullivan, q.v. SILLIMAN : V. under Seel(e)y,and -f- E. man,”
— Surnames of the United Kingdom: a concise etymological dictionary (2 vols in 1)