s_w / ((1 - 2 tr) sqrt(n)) with s_w^2 the Winsorized variance.
References
Wilcox, R. R. (2017). Modern Statistics for the Social and Behavioral Sciences, 2nd ed. Eqs (4.9)-(4.10).
Examples
TrimSE(c(2.1, 3.4, 1.9, 5.6, 4.4, 3.3, 2.8, 6.1, 3.9, 4.2, 2.2, 5.0, 40))$se
#> [1] 0.607161
