(a) Earlier in this Maseches (Shabbos 11a, also Pesachim 68b), RASHI
explained that "Igla Tilsa" is a cow's third offspring. Such a calf is well
developed and fat.
TOSFOS (Bechoros 19a DH d'Hach) questions the contention that the third
born is healthier than the first born, since the Gemara (ibid.) states that
a *firstborn* sheep is usually stronger and healthier looking. Rashi
(Shabbos 11a), however, apparently avoids this apparent contradiction by
explaining that a cow normally bears its first young when it is still weak
and relatively immature. By the time it bears its third brood, it is mature
enough to bear strong offspring. (The Gemara in Bechoros is discussing a
*mature* sheep that has not yet given birth.)
However, Rashi himself (Eruvin 63a) rejects this explanation based on a
Gemara in Sanhedrin which contends that every Erev Shabbos Rav Yochanan and
Reish Lakish would use the Sefer Yetzirah to create an "Igla Tilsa." How
can one "create" a calf that is the third to it mother! (Rashi in Sanhedrin
65b suggests that it just "tasted like" a healthy, third-to-its-mother
(b) RASHI in Eruvin 63a explains that "Igla Tilsa" is a third-grown calf
(one that has reached one-third of its full growth), which is particularly
tasty. (See also Rashi Bava Metzia 68a DH l'ch'she'Yiheyu Meshulashim).
However, as TOSFOS (Gitin 56a) points out, this clearly does not seem to be
the case with the "Igla Tilsa" of our Gemara, which was only 7 days old. A
calf certainly isn't one third grown after only 7 days!
(c) In a similar fashion, RASHI in Sanhedrin 65b explains that "Igla Tilsa"
is a calf that has reached one-third of its expected *lifespan*, at which
time it has reached its *full* growth. This explanation, however, suffers
the same problem as the previous one; how can a 7-day old calf have reached
one third of its expected lifespan?
(d) TOSFOS (Gitin 56a, Bechoros 19a) therefore concludes that "Igla Tilsa"
means a calf that is healthy and good tasting. The word "Tilsa" is not
related to the root "Telasa," meaning "three," but to the root "Shalish,"
which means "an appointee" (i.e., one that is worthy of being appointed
over others due to his personal prowess). Tosfos cites as examples of this
usage the verses "v'Shalishim Al Kulo" (Shemos 14:7, see Targum Onkeles)and
"Eglah Meshuleshes" (Bereishis 15:9 -- this is an original interpretation
of the verse, see also Chizkuni there).