Thelen
A surname most often explained as a shortened or Anglicized form of a Welsh patronymic, especially from Llewelyn.
The #6,065 most common surname in the United States (2010 Census) — about 1 in every 54,578 Americans shares it.
Etymology
Thelen is usually treated as a reduced form of the Welsh name Llewelyn, passing into a simpler spelling and pronunciation in English records. One 19th-century source explicitly explains it as meaning “the son of Llewelyn,” shaped by the difficulty English speakers had with the Welsh sound sequence. Because the name is quite short and the evidence is limited, this explanation is best viewed as a traditional derivation rather than absolute proof.
History
The name appears in English-language surname discussion as a rare form, and it was apparently understood by some writers as a nickname-like simplification of a Welsh patronymic. The modern surname is also found as a German family name, so some bearers may have separate continental origins rather than a single Welsh source. In the United States it remains uncommon.
Origin
Regions
Variant spellings
Fun fact
One old surname writer compared Thelen to Floyd for Lloyd: both were seen as spellings reshaped to fit English pronunciation.
From the historical record
“THELEN (Ger.) : v. the Appendix of Foreign Names. THELWALL \ (Eng.) Bel. toThelwall (Chesh.), THELWELL j 13th cent. rA^fea»e/= the Plank- Well [O.E. Yel, a plank -^a)(i>Ha, a well] 'Wall' is a Chesh. form of 'well.' THEOBALD (Teut.) People-Bold [O.Teut. Theod{p)bald, Thiodbald, Theotbald, etc.: V. under Theed, and…”
— Surnames of the United Kingdom: a concise etymological dictionary (2 vols in 1)